The application of coatings to metals to retard or inhibit corrosion is well established. This is particularly true in the area of metal food and beverage cans. Coatings are typically applied to the interior of such containers to prevent the contents from contacting the metal of the container. Contact between the metal and the packaged product can lead to corrosion of the metal container, which can contaminate the packaged product. This is particularly true when the contents of the container are chemically aggressive in nature. Protective coatings are also applied to the interior of food and beverage containers to prevent corrosion in the headspace of the container between the fill line of the food product and the container lid, which is particularly problematic with high-salt-content food products.
Packaging coatings should preferably be capable of high-speed application to the substrate and provide the necessary properties when hardened to perform in this demanding end use. For example, the coating should be safe for food-contact; not adversely affect the taste of the packaged food or beverage product; have excellent adhesion to the substrate; resist staining and other coating defects such as “popping,” “blushing” and/or “blistering;” and resist degradation over long periods of time, even when exposed to harsh environments. In addition, the coating should generally be capable of maintaining suitable film integrity during container fabrication and be capable of withstanding the processing conditions that the container may be subjected to during product packaging.
Various coatings have been used as interior protective can coatings, including epoxy-based coatings and polyvinyl-chloride-based coatings. Each of these coating types, however, has potential shortcomings. For example, the recycling of materials containing polyvinyl chloride or related halide-containing vinyl polymers can be problematic. There is also a desire to reduce or eliminate certain epoxy compounds commonly used to formulate food-contact epoxy coatings.
To address the aforementioned shortcomings, the packaging coatings industry has sought coatings based on alternative binder systems such as polyester resin systems. It has been problematic, however, to formulate polyester-based coatings that exhibit the required balance of coating characteristics (e.g., flexibility, adhesion, corrosion resistance, stability, resistance to crazing, etc.). For example, there has been a tradeoff between corrosion resistance and fabrication properties for such coatings. Polyester-based coatings suitable for food contact that have exhibited both good fabrication properties and an absence of crazing having tended to be too soft and exhibit unsuitable corrosion resistance. Conversely, polyester-based coatings suitable for food contact that have exhibited good corrosion resistance have typically exhibited poor flexibility and unsuitable crazing when fabricated.
What is needed in the marketplace is an improved binder system for use in coatings such as packaging coatings.